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By John Borrell/Sofia

It seemed a small thing, hardly ground for arrest. For two weeks a tiny group of Bulgarian environmentalists called Ecoglasnost manned a table in a Sofia park to gather signatures on a petition calling for public debate on two controversial river-diversion schemes. They had collected nearly 7,000 names, when police and militia units suddenly swooped down, scattered bystanders and arrested seven of the organizers.

The members of Ecoglasnost were later released, but the crackdown was a crude warning to Bulgarian political activists to watch their step. It was one more indication of just how nervous Eastern Europe's remaining hard-line regimes have become as a result of the year's dramatic political changes elsewhere in the bloc. The obdurate rulers in Czechoslovakia, Bulgaria and Rumania refuse to imitate their reformist neighbors but can't help looking anxiously over their shoulder. "They are all worried about the fallout from change elsewhere," said a Western diplomat in the region. A Bulgarian proverb captures the fears: "When the Gypsy's bear is dancing in your neighbor's yard, you know it will soon come to yours."

Although Sofia's police were frightened enough to rough up Ecoglasnost, which has just 101 members, Bulgarians have no modern model for revolt. That, ironically, might make gradual change easier. Czechoslovakia has such a model -- 1968's Prague Spring -- and authorities there are taking no chances. Two weeks ago, they arrested Jiri Ruml and Rudolf Zeman, well-known editors of the underground opposition newspaper Lidove Noviny. More than 100 journalists, most of them government employees, have since signed a petition calling for the release of the pair and for the immediate legalization of the newspaper. Now the government is hounding playwright Vaclav Havel, spokesman for the Charter 77 movement and the country's best-known dissident. Police called Havel in for questioning last Thursday, then allowed him to go to a city hospital when he complained of being ill. Their real purpose was to prevent him from taking part in unofficial celebrations Saturday to mark the 71st anniversary of the founding of the Czechoslovak state.

Earlier in the week six independent opposition groups had called for "quiet and solemn celebrations" throughout the country on the anniversary. Officials, fearing that the unauthorized gatherings could easily turn into giant antigovernment protests, sought to block them. To make sure that shops were well stocked during the week before the anniversary, authorities released onto the market large supplies of normally unobtainable imported bananas and oranges. "They continue to dangle these things in front of the populace as an incentive for political acquiescence," said a Western diplomat in Prague. "But it is clearly becoming harder and harder for them to buy off people in this way."

That does not mean that any of the remaining hard-line governments will necessarily be toppled anytime soon. Nor do they show signs of making more than minor changes in their orthodox programs. And there seems to be a flip side to Gorbachev's repudiation of the Brezhnev Doctrine: it also means that Moscow will not intervene to force reform. Intriguingly, though, some Soviet officials are debating whether it might be wiser to give a shove to the recalcitrant leadership in Czechoslovakia, where popular pressure for change seems ripest.

Nothing short of death seems likely to budge Rumanian leader Nicolae Ceausescu, who has maintained the most repressively Stalinist line while tending a personality cult and pursuing a Brobdingnagian building program. "Socialism," he told the ruling party's Central Committee this week, "is non-negotiable." Translation: Ceausescu's secret police will make sure that any challenge to his leadership is quickly snuffed out.

Communists in Bulgaria and Czechoslovakia are taking a much less rigid line. But in neither country are they prepared to concede the party's leading role in society, let alone contemplate legalized opposition groups. Their goal is to allow just enough political protest to prevent explosions but not enough to allow broad-based opposition groups to emerge.

It is a perilous high-wire act. Dissident groups like Bulgaria's Ecoglasnost readily admit that part of their agenda is to shake the party's hold on power. "Once you break this monopoly in one area, it will start crumbling everywhere," says one of the organizers, Deyan Kyurianov. But that is apparent to the bloc's remaining hard-liners too. The Gypsy's bear may not be kept away forever, but for the moment, he is dancing on a very short chain.